RE: EDM

This is looking like another one of those topics we make harder than it
really is.

I just threw together a mini-EDM and was able to blast some holes into some
metal. While it is far from what I would call a ready to use unit, I think I
am starting to understand that there is a lot of leeway in building an EDM.

Here is what I set up:
Transformer 48 V 10 amp
Bridge Rectifier 400 v 35 amp
Filter cap 1900 mfd 450 V
Resistor network 8 - 13 ohm 50 watt wired up to yield 6.5 ohm 400 watt (if I
figured right)
Spark cap 500 uf 330v photo flash electrolytic (I know, I was warned, but it
was on hand and the HSM EDM book tells you to use electrolytics designed for
TV & radio & switching power supplies. I had the flash units on hand and put
it in a bunker made out of transformers and aluminum blocks incase it blew
up!)

For an electrode I used a piece of brass tubing about 1/8" dia and I used
some Tap Magic aluminum formula as a fluid and held the electrode in a pair
of insulated pliers. I turned on the power and brought the electrode to the
work piece and sparks started flying!! It is VERY hard to control the gap
correctly with the electrode hand held, but I was able to blast a hole about
a 1/8" deep before I got bored.

So, for my next trick I am going to get some mylar caps around .5 - 2 uf and
make up a mount like Jon Elson did so I can use the quill of my hill head to
gain better control over the distance.

Tim
[Denver, CO]

> -----Original Message-----
> From: Scott Acorn [mailto:sacorn@...]
> Sent: Saturday, July 17, 1999 7:09 PM
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: Re: [CAD_CAM_EDM_DRO] EDM
>
>
> From: Scott Acorn <sacorn@...>
>
> some more info.
> I suggest you use a step down transformer --120v to 60v
> after you run the 60v ac threw a 20 amp full wave rectifer and a
> 2000uf cap.
> you will end up with aprox. 80v dc----because of rms
> 98% of the edms use this voltage--why?---because a higher
> voltage=increased
> electrode where.
>
> My power supply can provide up to 9 or 10 amps.This is limited by ballast
> resisters---200w
> Most jobs use 5amps for rufing and 2-3 amps for finishing.
>
> Caps---.1uf for fine finishes
> a number of .3uf 5 i think
> all caps can be connected in parallel by switches to change power
> they are myler 400v
> DO NOT USE ELECTROLYTIC TYPE CAPs
> they cannot stand up to the high charge and discharge cycles --good for
> the 4th. of july but not good for soft flesh.
>
> on off times-- In order to obtain the lowest wear it is nessary to control
> how long the discharge lasts in usec. and how long for the next
> discharge.
> An example for low wear would be 100usec. on--25usec.off
> In my machine this is controlled by mosfets.
> we will get into this in more detail latter.
>
> Details of op amp ckt.
> It consists of 2comparators using a 1458 chip .
> The voltage across the gap is compared to ref. voltages
> set on the machine.
> A high limit and a low limit
> this in turn tells the stepper motor driver to advance or retract
> the electrode.
> The gap between elec. and work being controlled and maintained at around
> .001 to .002
> Tim Goldstein wrote:
>
> > From: "Tim Goldstein" <timg@...>
> >
> > Scott,
> >
> > Thanks for the quick reply and info.
> >
> > Here are the questions that come to mind.
> >
> > You suggest a 60 v power supply. What happens if you up the voltage?
> >
> > What type of amperage does your power supply provide?
> >
> > What type of caps are you using for the spark and how large a value?
> >
> > I am not following what you mean by adjustable on/off times.
> >
> > By adjustable power setting do you mean the capacitance or the
> voltage or
> > both?
> >
> > Can you give details of the op amp circuit that controls the
> steppers? I am
> > assuming it outputs a step and direction signal to the stepper
> controller
> > board?
> >
> > Where do you get the graphite for the electrodes and the EDM
> oil from? Any
> > suggestions for a source that sells small quantities?
> >
> > Thanks in advance for you help in this.
> >
> > Tim
> > [Denver, CO]
> >
> > ----- Original Message -----
> >
> > > From: Scott Acorn <sacorn@...>
> > >
> > > Hi Tim
> > > Well its a long story !
> > > The machine i have is the result of about 5yr. of design and dev.
> > > This machine will do some very fine work and i use it for making
> > > plastic injection molds.
> > > It has adj. on off times as well as adj. power settings.
> > > The ram is powered by a small stepper with a small belt
> reduction set up,
> > > so each step moves the ram down a very small amount.
> > > A op amp ckt. looks at the voltage across the gap while the machine is
> > cutting.
> > > this in turn tells the ram stepper to go lower or to backup.
> > > I can keep yoking but it may be boring if i do.
> > > so if you would like any other info. just ask--- if the
> interest is their
> > i can answer
> > > just about any question on edm that you might have.
> > >
> > > Pit falls and dangers
> > > use only edm oil in your mach. ------Fire
> > > only edm in a sumerged tank of oil with a min. of 1" oil covering part
> > > edm power supplies---use a step down transformer 60v is ideal.
> > > best electrode mat. ---- graphite
> > >
> > > I guess the bottom line would be ----edm mach. can be very
> simple or very
> > > complex---it all depends on the work you plan to do.
> > >
> > > Scott Acorn
>